Imaging Questions & Answers

Why turn to InGaAs for NIR detection?

InGaAs is an alloy which belongs to the InGaAsP quaternary system that consists of indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), and gallium phosphide (GaP). These binary materials and their alloys are all III-V compound semiconductors.

The energy bandgap of InGaAs alloys depends on the ratio of indium and gallium content. At room temperature (300 K), the dependency of the energy bandgap on the indium content x (0~1) can be calculated using the formula: Eg(x) = 1.425eV - 1.501eV*x + 0.436eV*x2. The corresponding cutoff wavelength that can be detected is in the range of 870nm~3.4µm.

Indium Content x Energy Gap Eg eV Corresponding Wavelength nm

What is InGaAs “standard wavelength” or “extended wavelength”?

The most used substrate for InGaAs is InP. The InGaAs alloy having x=0.530 has the same lattice constant as InP, which is called "standard InGaAs." This combination brings high quality thin films and results in the cutoff wavelength of 1.7µm.

However, many applications require longer wavelengths. Hamamatsu offers both linear and area InGaAs image sensors with cutoff wavelengths up to 2.6µm, which are called “extended wavelength.” Due to the mismatch of the lattice constant of InGaAs and InP, the quality of the thin films is reduced. However, Hamamatsu put in a lot of effort to guarantee top-quality extended InGaAs.

How can we suppress the dark current of InGaAs image sensors?

The dark current of Hamamatsu InGaAs image sensors is successfully minimized by operating the photodiode array at zero bias condition. Moreover, one-stage TEC (thermoelectric cooler) or multiple-stage TEC can be added into the sensor package to stabilize the sensor temperature and reduce the dark current efficiently.

There is no light to my camera, but I still have signal. What does this mean?

Rewording this question into camera terms, we can say, “The input to the camera sensor is blocked from detecting any photons, but the image data on my computer has non-zero values.”

This is an important feature of a scientific digital camera used for quantitative image measurements. To understand why this is the case we need to understand, in very high level terms, the conversion of photons to image data. The sensor detects the photons which are collected as photoelectrons and then passed along as a voltage in the readout circuit of the sensor. This voltage goes into a digitizer, which outputs a value represented by a whole number ranging from 0 to the maximum value of the digitizer. This whole number is referred to as counts, gray values, or gray levels.

The readout of the sensor pixel is an imperfect process and noise is introduced into the signal as it is converted to a voltage reading. This noise is a small fluctuating voltage around the nominal signal. If that signal is 0, then the voltage fluctuates into negative values. Since the digitizer in the camera does not contain values less than zero, these negative voltages would be clipped and data would be lost. To avoid the loss of data, the camera designer will set the zero voltage to be a number higher than zero that will accommodate the noise fluctuation, for example 100 counts on the digitizer. In this case, fluctuations below 0 in voltage would be represented by output counts less than 100 counts.

This non-zero output value for the zero photon input is called the digital offset. The camera manual or camera manufacturer can provide the digital offset number for your camera model. You will need to subtract this digital offset number from each intensity value to determine the true output signal from your camera.

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Meet the engineers

Lu Cheng is an applications engineer, specializing in all of our image sensors and driver circuits. Before joining Hamamatsu, she worked as an analog and mixed-signal ASIC designer. With more than 10 years’ experience in ASIC and circuit board design, she can support you to find the optimized product/solution not only from a user’s perspective but also from a designer’s. Traditional Chinese dancing is one of the things that make her learn about the beauty of the world from a different perspective.

Shelley Brankner is an Applications Engineer specializing in scientific cameras and x-ray imaging products. For customers that need high-level synchronization between their camera and peripheral devices, she can provide the expertise on timing and modes of operation in these imaging products. She has a passion for asking the question, “How does that work?” and a desire for sharing the answer with others. When she isn’t knocking down the technical questions that cross her path, she can be found knocking down pins at a bowling alley.

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